The present invention relates to a process and apparatus for the production of containers, e.g., jars, cups, bottles, etc., from plastic material, such as polyethylene terephthalate (PET), including optionally adding barriers during the molding process to impart special properties.
Many of these plastic containers must be adequately impermeable to moisture, alcohol, CO.sub.2, oxygen, etc., and also be able to withstand pressurization where required. They may be used, for example, for storing soft drinks, beer, fruit juices, and the like. Another important requirement for these containers is that the molded plastic should not give off an offensive odor or taste.
However, when molding PET, it is very difficult if not impossible to avoid the presence of acetaldehyde (AA) which does impart a disagreeable taste to the stored product. Unfortunately, AA is an unavoidable by product when heating PET, and the amount generated is directly related to the heat history of the plastic from its manufacture to the final blowing operations. Whenever the PET is heated, the amount of released AA increases. The industry specifies maximum allowable limits for the presence of AA in PET containers.
In conventional methods of injection molding and blow molding processes for these products, the plastic during its manufacture is hot, then extruded, cooled and pelletized into granules which are supplied to the injection molding machine, where the plastic is again heated in the extruder for the molding of the preforms.
In the one-stage method, after molding the not fully cooled preforms are partly heated and their temperature stabilized as required. The preforms are then transferred to a blow station for final processing. In the two-stage method, the preforms are cooled completely after molding. They are then reheated and stabilized for final blow molding. In both of these methods, the preforms are heated from the outside only, and must be heated above the required blowing temperature so that the plastic can stabilize at the required temperature with the temperature distributed evenly from the outside to the inside. Either method generates a considerable heat history for the plastic, and results in higher and more difficult to control AA levels, and can easily lead to rejects.
It is important to consider that a harmful heat history requires starting the process of making the container with a substantially more expensive polymer than that which is required to meet the properties of the final container. Thus, it is particularly advantageous to start with substantially the same quality of polymer as will be found in the finished container. A higher grade of polymer is substantially more costly than that which is the result of degradation due to the use of the conventional process, i.e., injection stretch blow molding.
Accordingly, it is a principal object of the present invention to provide a process and apparatus for conveniently, expeditiously and economically forming a plastic container.
It is a further object of the present invention to provide a process and apparatus as aforesaid which forms a container from a preform precursor and preform in a continuous operation.
It is a still further object of the present invention to provide a process and apparatus as aforesaid which substantially reduces the heat history of the plastic and reduces the AA content.